staging: unisys: add comment to spinlock in struct charqueue

Add a comment to the charqueue's spinlock to explain that it is a lock for the
structure.

Signed-off-by: Ken Depro <kenneth.depro@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>